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Will a wall with spray foam and 1.5" exterior Roxul have condensation issues?

I am planning a gut-redo/addition to my house. I would like to place rigid insulation on the exterior but I am limited to 1.5" (local zoning laws force me to keep the existing side walls else the structure shrinks in width by 5 feet - to comply with new rules). Because I can only get 1.5" I am thinking of using 2lb foam inside the studs (to R-30) and Roxul ComfortBoard IS on the exterior (to reduce thermal bridging and allow the sheathing to dry to the outside).

But with only 1.5" of exterior insulation the sheathing will remain cold. Is there a concern of condensation on the sheathing? I should note that I am planning to air seal the entire house at the wall/roof sheathing (also cover the roof in rigid insulation), hopefully to 1 ACH @ 50 Pa. I am in Toronto, Canada which I believe is climate zone 6

Asked by David Cook
Posted Sun, 02/02/2014 - 21:05
Edited Mon, 02/03/2014 - 08:57


1 Answer

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Q. "Will a wall with spray foam and 1.5 inch exterior Roxul have condensation issues?"

A. No. The closed-cell spray foam is an air barrier and a vapor retarder, and will prevent any interior moisture from reaching the wall sheathing. Moreover, this type of wall can dry to the exterior if the sheathing ever gets damp.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 02/03/2014 - 09:01

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